This article explains how to host an HBO Max watch party using the Teleparty or Scener browser extension. Both groups (host and watchers) need to use the same extension and have a current HBO Max subscription.
How to Host an HBO Watch Party
HBO Max doesn't have a built-in feature like Prime's Watch Party, which lets you host virtual viewings with friends and family. In HBO Max's case, you use a browser extension to watch the same HBO Max movie or TV Show online with family and friends.